Ceramic bodies are useful in a wide variety of applications including, for example, filtration of diesel and gasoline engine exhaust, and other applications, such as membrane separations and flow-through catalytic converters. By way of non-limiting example, ceramic honeycomb structures or bodies may be selectively plugged and used as diesel particulate filters (DPFs) and gasoline particulate filters (GPFs). Exhaust gas flows into open inlet channels of the DPF or GPF, through the porous wall of the honeycomb (because the inlet channels are plugged at the other end) and out of the outlet channels (which are plugged at the inlet end). During exhaust gas passage through the porous honeycomb wall, small particulates from the exhaust gas can be deposited on the pore surface or as a soot layer on the wall surface, thus providing filtering of the exhaust gas. The soot cake of deposited particulates may be periodically burned off in a regeneration cycle or continuously during passive regeneration such that the lifetime of the DPF or GPF matches that of the vehicle.
Tightening of exhaust gas regulations may call for higher particulate filtration efficiency, particularly for small particles, and for higher NOx filtration efficiency, not only in currently established test cycles, but also in continuous real-world application. Moreover, the desire for filters with lower pressure drop, improved thermal shock resistance, and extended lifespan may necessitate new ceramics with higher porosity, larger pore size, and thinner honeycomb walls. For instance, DPFs and GPFs should have high filtration efficiency for emitted particles and high porosity to allow gas flow through the walls without restricting engine power, while at the same time demonstrating low back pressure. The filters should also be able to withstand the erosive and corrosive exhaust environment and bear thermal shock during rapid heating and cooling.
Ceramics can be formed by reactive sintering of raw materials via a sequence of solid and/or liquid state reactions with various intermediate products. Such sintering reactions tend to yield ceramics with wide grain size distributions and, in the case of multi-phase ceramics, inhomogeneity in the microstructure, including the distribution of the main phase, minor phase, and intergranular and triple point pocket glasses. Microstructural inhomogeneity can affect ceramic properties, such as thermo-mechanical properties. For instance, cordierite and aluminum titanate ceramic bodies may exhibit anisotropy in their thermal expansion with different crystallographic directions exhibiting positive and negative expansion. Due to the anisotropy in thermal expansion, mismatch strains can build up between grains with different crystallographic orientation and such strains can lead to microcracking which, in turn, can lead to decreased material strength and fracture toughness. Thus, observed ceramic material performance is often far below the theoretical performance predicted based on single crystal phase behavior.
Increased microstructural homogeneity can be improved using glass-ceramic methods, in which raw materials are first heated to form a glass melt, which is then cooled into a glass that is subsequently cerammed in an annealing step during which it crystallizes and forms a glass-ceramic. The crystallite sizes and fractions of the resulting glass-ceramics can depend on, e.g., glass composition and annealing conditions, such as temperature, time, environment, cycles, applied stress, and temperature gradient, to name a few. Ideally, a glass-ceramic can comprise a fully or mostly crystallized material (e.g., a ceramic showing only some residual glass in grain boundaries and/or triple junctions).
While glass-ceramics may exhibit relatively higher microstructural homogeneity as compared to ceramics produced by reaction sintering, current glass-ceramic processes may be limited from a compositional standpoint due, for instance, to temperature limitations. To melt the raw materials in the first step of the process, the melt is typically brought to a temperature about 100° C. in excess of the melting point of the composition. Thus, it can be difficult to melt mixtures having a glass transition temperature (Tg) greater than about 1500-1600° C. Because of these limitations, it is currently difficult, if not impossible, to generate glass-ceramics from certain raw materials, particularly batch materials with a higher Tg.
Accordingly, it would be advantageous to provide methods for forming glass-ceramics with improved microstructural homogeneity, larger grain size, higher porosity, and/or larger pore size without strongly promoting or completely suppressing microcracking or sacrificing other properties such as material strength and/or fracture toughness. Moreover, it would be desirable to provide glass-ceramic methods suitable for processing unconventional raw materials, e.g., materials with a high Tg. The resulting glass-ceramics can have improved homogeneity and thus improved strength, while also exhibiting increased porosity and/or increased pore size.